What do you really want to do with your career?

If it's airlines, then you need to go to a regional. I would probably avoid the bottom-feeders with high wash-out rates, you don't need a 121 pink slip.

If it's corporate, you need to network. With 4000+ hours, you should already know some people.

If it's just not happening in 91/135, or the lack of turbine time is clearly causing you to miss opportunities then a year or two at a regional might help, but obviously maintain your 91 network while you're doing that. Be advised that if you think you need TPIC, the wait could be years at a better regional. For fast upgrade, you will need a bottom feeder (with low pay, low QOL, high washout rate, etc). If you think TSIC will do the trick, then a quick trip through a regional might work.

Contrary to what some other folks have said, I do know a G-V pilot who hated the whole idea of the regionals and worked his way up in 91/135. But he says he wished he had just done the regionals up front to get the turbine time...he thinks it would have saved him 8-10 years.